It should work in many / most cases, since your users should be on the same cluster of mail machines most of the time[1] -- but it's not totally safe. If you don't mind that, referencing /home/USER/sausr/local/bin/spamassassin (where USER is the user spamassassin is installed under) *should* work. This will only work for shell / ftp users, not for mail-only users (since you have no way of editing their .procmailrcs).

If another user is on the same group of mail machines but a different user machine, they won't be able to use "sa-learn" and other training commands.

I think with the junk filter providing a lot of the same functionality (albeit in a slightly less effective way, and with less control), DH is probably not wild about the idea of encouraging people to setup SA, except on an individual "per-account" basis. Hopefully, they'll eventually add functionality for adaptive filtering (with the bayes-sql module) into the junk filter itself.

Side note... once the Sarge upgrades happen on the mail machines as well, or if you don't mind not being able to train the filter, using .db files should work Ok.

[1] Unless things have changed recently, users / domains on the same account will generally be provisioned on the same machine or group of machines, however users / domains across accounts may be on totally different "clusters".